Re: [HACKERS] Race conditions with WAL sender PID lookups

On 2017-05-21 06:37, Erik Rijkers wrote:
> On 2017-05-20 14:40, Michael Paquier wrote:
>> On Fri, May 19, 2017 at 3:01 PM, Masahiko Sawada 
>> <sawada.mshk@gmail.com> wrote:
>>> Also, as Horiguchi-san pointed out earlier, walreceiver seems need 
>>> the
>>> similar fix.
>> 
>> Actually, now that I look at it, ready_to_display should as well be
>> protected by the lock of the WAL receiver, so it is incorrectly placed
>> in walreceiver.h. As you are pointing out, pg_stat_get_wal_receiver()
>> is lazy as well, and that's new in 10, so we have an open item here
>> for both of them. And I am the author for both things. No issues
>> spotted in walreceiverfuncs.c after review.
>> 
>> I am adding an open item so as both issues are fixed in PG10. With the
>> WAL sender part, I think that this should be a group shot.
>> 
>> So what do you think about the attached?
> 
>> [walsnd-pid-races-v3.patch]
> 
> 
> With this patch on current master my logical replication tests
> (pgbench-over-logical-replication) run without errors for the first
> time in many days (even weeks).

Unfortunately, just now another logical-replication failure occurred.  
The same as I have seen all along:

The symptom: after starting logical replication, there are no rows in 
pg_stat_replication and in the replica-log logical replication complains 
about max_replication_slots being too low. (from previous experience I 
know that making max_replication_slots higher does indeed 'help', but 
only until the next (same) error occurs, with renewed (same) complaint).

Also from previous experience of this failed state I know that it can be 
'cleaned up' by
manually emptying these tables:  delete from pg_subscription_rel;  delete from pg_subscription;  delete from
pg_replication_origin;
Then it becomes possible to start a new subscription without the above 
symptoms.

I'll do some more testing and hopefully get some information that's less 
vague...


Erik Rijkers